The asymmetric terphenyl-2'-carboxylate ligand 3,5-dimethyl-1,1':3',l ''-terphenyl-2'-carboxylate, (-O2CArPh,Xyl), was prepared in high yield. This ligand facilitates the assembly of the diiron(II) complexes [Fe-2(mu-O2CArTol)(2)(O2CArPh,Xyl)(2)(THF)(2)] [2, (-O2CArTol) = 2,6-di-p-tolylbenzoate], [Fe-2(mu-O2CArTol)(2)(O2CArPh,Xyl)(2)(pyridine)(2)] (5), [Fe-2(mu-O2CArPh,Xyl)(2)-(O2CAr (Ph,Xyl))(2)(THF)(2)] (3), and [Fe-2(mu-O2CArPh,Xyl)(2)(O2CArPh,Xyl)(2)(pyridine)(2)] (6), all of which have a windmill geometry. The iron-iron distance of 3.355[10] (A) over circle in 6 is similar to 1 (A) over circle shorter than that in the analogue [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(pyridine)(2)] (4) and similar to the similar to 3.3 (A) over circle metal-metal separation at the active site of the reduced diiron(II) form of the soluble methane monooxygenase hydroxylase enzyme (MMOHred). A series of ortho-substituted picolyl-based ligands, 2-picSMe, 2-picSEt, 2-picS'Bu, 2-picSPh, 2-picSPh(Me-3) (Ph(Me-3) = mesityl), and 2-picSPh('Pr-3) (Ph('Pr-3) = 2,4,6-triisopropylphenyl), were prepared and allowed to react with [Fe-2(mu-O2CAr)(2)(O2CAr)(2)(THF)(2)] to produce [Fe-2(mu-O2CAr)(3)(O2CAr)(picSR)] (7-13, Ar = Ar-Tol or Ar-Ph,Ar-xyl) complexes in 45-87% yields. The substrates tethered to the pyridine N-clonor ligands picSR, where R = Me, Et, 'Bu, or Ph, coordinate to one iron atom of the diiron(II) center by the nitrogen and sulfur atoms to form a five-membered chelate ring. The Fe-S distance becomes elongated with increasing steric hindrance imparted by the R group. The most sterically hindered ligands, 2-picSPh(Me-3) and 2-picSPh('Pr-3), bind to the metal only through the pyridine nitrogen atom. The reactions of several of these complexes with dioxygen were investigated, and the oxygenated products were analyzed by H-1 NMR spectroscopy and GC/MS measurements following decomposition on a Chelex resin. The amount of sulfoxiclation product is correlated with the Fe center dot center dot center dot S distance. The ratio of oxidized to unoxidized thioether substrate varies from 3.5, obtained upon oxygenation of the weakly coordinated 2-picSPh ligand in 10, to 1.0, obtained for the bulky 2-picSPh(Pr-3) ligand in 12, for which the iron-sulfur distance is >4 (A) over circle. External thioether substrates were not oxidized when present in oxygenated solutions of paddlewheel and windmill diiron(II) complexes containing 1-methylimidazole or pyridine ligands, respectively.